U.S. Pat. Nos. 5,626,191 and 6,412,557, incorporated herein in their entirety, disclose in situ combustion processes for producing oil from an underground reservoir (100) utilizing an injection well (102) placed relatively high in an oil reservoir (100) and a production well (103-106) completed relatively low in the reservoir (100). The production well has a horizontal leg (107) oriented generally perpendicularly to a generally linear and laterally extending upright combustion front propagated from the injection well (102). The leg (107) is positioned in the path of the advancing combustion front. Air, or other oxidizing gas, such as oxygen-enriched air, is injected through wells 102, which may be vertical wells, horizontal wells or combinations of such wells. The process of U.S. Pat. No. 5,626,191 is called “THAI™”, an acronym for “toe-to-heel air injection” and the process of U.S. Pat. No. 6,412,557 is called “Capri™”, the Trademarks being held by Archon Technologies Ltd., a subsidiary of Petrobank Energy and Resources Ltd., Calgary, Alberta, Canada.
High-Pressure-Air-Injection, HPAI, is an in situ combustion process that is applied in tight reservoirs containing light oil. In these reservoirs, a liquid such as water cannot be effectively injected because of low reservoir permeability. Air is injected in the upper reaches of the reservoir and oil drains into a horizontal well placed low in the reservoir. The process provides some heat by low-temperature oil oxidation and more importantly, it provides pressure-maintenance to enable high sustained oil rates. This process can be applied in any reservoir that contains oil that is mobile at reservoir conditions.
Of concern is the safety of the THAI™ and Capri™ processes with respect to oxygen entry into the horizontal well, which would cause oil burning in the well and extremely high temperatures that would destroy the well. Such oxygen breakthrough will not occur if the injection rates are kept low, however, high injection rates are very desirable in order to maintain high oil production rates and a high oxygen flux at the combustion front. A high oxygen flux is known to keep the combustion in the high-temperature oxidation (HTO) mode, achieving temperatures of greater than 350° C. and combusting the fuel substantially to carbon dioxide. At low oxygen flux, low-temperature oxidation (LTO) occurs and temperatures do not exceed ca. 350° C. In the LTO mode, oxygen becomes incorporated into the organic molecules, forming polar compounds that stabilize detrimental water-oil emulsions and accelerate corrosion because of the formation of carboxylic acids. In conclusion, the use of relatively low oxidant injection rates is not an acceptable method to prevent combustion in the horizontal wellbore.
What is needed is one or more methods to increase the oxidizing gas injection rate while preventing oxygen entry into the horizontal wellbore. The present invention provides such methods.